1. Field of the Invention
The present invention relates to apparatus for automatically measuring the spherical refractive power and astigmatism of eyes.
2. Description of the Prior Art
Refractometry is essential for early detection of trouble in eyes and preparation of corrective lens. There is an increasing demand for apparatus which enables measurement of eye refractive power with higher accuracy and speeds.
Various types of automatic eye-refractometers have already been proposed to satisfy the requirements mentioned above and some of them are already known in the art. See, for example, U.S. Pat. Nos. 3,819,256 and 4,021,102 and Japanese Patent Application laid open Nos. 52,893/1979 and 77,495/1979 (U.S. Application Ser. No. 944,304, now U.S. Pat. No. 4,293,198, issued Oct. 6, 1981). Among them, the last-mentioned three prior inventions have direct relevance to the subject of the present invention. According to the prior inventions, measuring marks extending in three different directions are projected through the pupil onto the fundus of an eye to be examined and the images of the marks reflected from the fundus are detected relative to three meridians to measure the refractive power of the eye. Spherical visibility, degree of astigmatism and axis of astigmatism of the examined eye are found from the measured result employing the general formula of refractive power, y=a sin (2.theta.+.zeta.)+b.
Since, as is well-known to those skilled in the art, the reflecting power of fundus is very low, the reflected light from the fundus is very weak as compared with the incident light. Therefore, the accuracy of measurement using such reflected light is greatly affected by noise.
Light reflected upon the cornea constitutes a source of such noise. If the reflected light from the cornea is mixed into the measuring light reflected by the fundus, the measuring accuracy is remarkedly reduced thereby. Therefore, it is desirable to exclude the light reflected from the cornea. The accuracy of measurement is reduced also by change in size of the pupil of the optical system of the refractometer. The size of the pupil may change during power adjusting (focusing). U.S. Ser. Nos. 65,004 now U.S. Pat. No. 4,318,585; 75,115 now U.S. Pat. No. 4,372,655 and 131,499, now U.S. Pat. No. 4,376,573 have proposed a solution to this problem.
In some embodiments described in the aforementioned prior applications, the refractive power of the examined eye is measured by detecting the displacements of images of three target marks reflected from the fundus of the eye. To successfully carry out the measurement, the areas on the cornea of the subject eye through which the target mark constituting beams pass respectively, have to be separated from each other to prevent mutual interference of respective detected information. Therefore, the individual area through which one target mark beam passes is reduced to the extent that insufficient light is reflected from the fundus. Also, as another disadvantage of the prior art devices, the inventors of the present invention have found that when a linearly elongated target mark is projected upon a fundus and an image of the target reflected from the fundus is detected, the target mark image is deformed at both of its ends due to the astigmatism of the examined eye and there is caused thereby a detection error the degree of which corresponds to the degree of the astigmatism.